JPH0441915B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to optical recording media, particularly erasable optical recording media.

(Prior Art) The optical recording method, in which a laser beam is focused on a minute spot by a condensing lens and irradiated onto the surface of an optical recording medium to cause an optical change on the medium and record information, is a high-density information recording method. This method is attracting attention as a possible method. A wide variety of media have been proposed and considered for use in the optical recording. Particularly erasable optical recording media according to the present invention include so-called magneto-optical recording media using magneto-optical materials that exhibit magneto-optical effects such as the Faraday effect and Kerr effect, and those that exhibit optical properties due to phase changes of materials such as chalcogenide compounds. A recording medium using so-called phase change that causes a difference is known as a typical recording medium. In addition to this, various other media have been proposed, and information recording media using polymeric liquid crystals, similar to the present invention, have also been proposed (Japanese Patent Application Laid-open No. 59-10930, Japanese Patent Application Laid-open No. 59-35989).

(Problems to be solved by the invention) Magneto-optical recording media are currently the most powerful recording media and are close to being put into practical use, but their S/N is generally as low as ~40 dB, and the external voltage applied to the media is low. Since the response speed of the magnetic field is slow, partial erasure of each bit, which is the recording unit of information, is not possible, and since one track is erased all at once, rewriting of information becomes complicated. Furthermore, the materials currently being considered as recording media that utilize phase change are Te-based oxides or Te-based alloys, but these have problems with the toxicity of the materials and are feared to have an adverse effect on the human body. Further, these recording media have the disadvantage that they are made of materials that can be made thin by techniques such as vapor deposition or sputtering, and that they are expensive to manufacture. There is a strong desire for an optical recording medium that is simple to manufacture and inexpensive. As mentioned above, a recording medium using a polymer liquid crystal has been proposed as a way to solve the above-mentioned drawbacks. Since it is used in this state, the incident light is scattered and the detection mechanism is limited, so it has the disadvantage that the S/N ratio is not necessarily sufficient.

(Means for Solving the Problems) The present invention aims to improve the above-mentioned drawbacks, that is, to provide an erasable recording medium that is easy to manufacture, low cost, and can realize a high S/N ratio. For this purpose, an optical recording layer made of a cholesteric polymer liquid crystal layer containing a light absorber is provided, and the helical pitch P and refractive index n are approximately λ=np with respect to the reading light source wavelength λ.

(Operation) The basic operating principle of the present invention will be explained. A cholesteric polymer liquid crystal is used for the recording layer. It is known that liquid crystal groups in cholesteric polymer liquid crystals have a helical arrangement, and because of the helical arrangement, they exhibit optical properties unique to cholesteric. A typical optical property is a wavelength-selective reflection phenomenon of light corresponding to the periodic pitch of the helical structure. This is a phenomenon in which, assuming that the pitch of the cholesteric polymer liquid crystal is p and the refractive index is n, only circularly polarized light having a wavelength λ=np and rotating in the same direction as the helical rotation direction is selectively reflected. The present invention is based on this selective reflection phenomenon. The light selective reflection wavelength can be changed by changing the refractive index and pitch of the polymer liquid crystal. It has been known that the pitch of low-molecular-weight cholesteric liquid crystals changes depending on temperature and other factors.

As a result of intensive studies by the present inventors, it has been found that when a cholesteric polymer liquid crystal in a cholesteric phase is rapidly heated by laser beam irradiation, the color changes due to a change in the light selection wavelength that seems to correspond to a change in the helical pitch length or rotation of the helical axis. It has been found that when a change (transmittance change) occurs and rapid cooling is achieved by removal of laser beam irradiation, the color change state is preserved. This is thought to be because the changed pitch state was preserved as it was due to the rapid cooling effect. On the other hand, it has been found that when slowly cooled, the color change state disappears and returns to the original state. When the irradiation energy of the laser beam is further increased, unlike the color change corresponding to the change in pitch length, pits are formed and preserved by rapid cooling, and the pre-formed pits disappear by reheating and slow cooling, returning to their original state. confirmed to return.

The present invention will be explained in more detail with reference to the drawings. 1st
The figure is a schematic cross-sectional view of an embodiment of the optical recording medium of the present invention. In FIG. 1, a polymeric liquid crystal layer 2 is formed on a plastic or glass substrate 1, which contains a light absorber that efficiently converts the writing light beam into heat. The light absorber is preferably one that has large absorption in the wavelength range of the light beam and has a relatively high melting point.

Semiconductor laser (λ = 0.78 ~
0.83 μm), phthalocyanine compounds such as vanadyl phthalocyanine can be used. Various types of cholesteric polymer liquid crystals can be used. One example is a siloxane-based polymer liquid crystal to which a cholesterol derivative and nematic liquid crystal molecules are added, as shown by the following structural formula [].

In the present invention, the cholesteric liquid crystal contains the light absorbent. Further, a trace amount of additives such as a plasticizer may be included in order to improve film-forming properties. Light absorbers can be included in the cholesteric liquid crystal in various forms. For example, it is sufficient to simply disperse the light absorbent. However, in general, it is not possible to obtain a uniform polymeric liquid crystal layer by dispersing enough light to absorb light, so there are restrictions on the amount of dispersion. Therefore, as in the present invention, if a light absorbing agent such as the above-mentioned panadyl phthalocyanine is added-polymerized to a polymer such as siloxane or vinyl polymer, for example, the following compound () may be used. By increasing the substantial amount of the light absorbing agent added, the light absorption effect becomes greater and a uniform polymeric liquid crystal layer can be formed. A similar effect can be obtained by copolymerizing a unit having a liquid crystal group and a unit having light absorption properties, such as the following compound [].

The polymer liquid crystal film can be made into a thin film by various methods. The polymer liquid crystal can be solubilized in a suitable solvent and applied by spin coating, etc., transferred by gravure printing, applied with a doctor blade, etc., and the applied film can be made into a thin film by heating and drying. can. Alternatively, it is also possible to form a thin film by molding it on a substrate under heat and pressure.

The film thickness of the polymer liquid crystal thin film is 0.1 μm to several tens of μm.
is adjusted to A protective film or protective layer 3 is generally formed on the polymer liquid crystal film. However, the protective film 4 is not an essential requirement of the present invention, and may be omitted. The helical rotation direction of the polymer liquid crystal is determined by the optically active material used. A polymeric liquid crystal containing a cholesterol derivative as shown in the above structural formula [1] is selectively reflected around a wavelength shown by λ=np of left-handed circularly polarized light.

Therefore, when the selective reflection wavelength of the pitch is adjusted from the visible range to the near-infrared wavelength range, and when circularly polarized light rotated in the same direction as the cholesteric light is used as the readout light 4, an extremely large optical change occurs, resulting in a high S/N can be achieved. That is, the pitch p
If read out and adjusted so that λ is approximately equal to np with respect to the light source wavelength λ, the area where no information is written will have an extremely high reflectance, while the information will be written in a mode that forms a pit. In this region, there is a rapid decrease in reflectance, resulting in high S/
N can be realized. On the other hand, if the pitch p is adjusted in advance to ≫np with respect to the wavelength λ of the reading light source, and information is written in a light amount region where the pitch length becomes long, the non-information writing area has an extremely low reflectance. The writing section has a high reflectance, and a high S/N ratio can be achieved as well.

The recording layer can be easily and simultaneously formed in large quantities by methods such as coating and printing. In addition, the production time is less than 1/20 that of vapor deposition, making it excellent for mass production.
The material cost is less than 1/10 compared to inorganic materials such as Te.
Lower cost.

Example 1 Polymer liquid crystal shown in compound [] and vanadyl phthalocyanine added polymer shown in compound [] were dissolved in a solvent and applied by spin coating on a glass substrate, and dried to form a polymer containing a light absorber. A liquid crystal layer was formed. The selective reflection center wavelength of the polymer liquid crystal was adjusted to approximately 830 nm. A laser diode with a wavelength of 830 nm was used as the light beam. 8m
The formation of pits was confirmed by writing with pulsed light of W80 μS. When the medium was reproduced with 1 mW of left-handed circularly polarized light, reproduction was possible with S/N = 60 dB. By heating the medium on which the information was written to 70°C or higher, the information could be completely erased. Note that no deterioration of the medium was observed.

Example 2 Compound as polymer liquid crystal containing light absorber []
When light irradiation was carried out in the same manner as in Example 1 using a medium having the same configuration as in Example 1, almost the same results as in Example 1 were obtained.

Example 3 A medium with the same configuration as Example 1 except that the selective reflection wavelength of the polymer liquid crystal was adjusted to 550 nm.
Writing was performed using pulsed light with a width of 80 μS, and color changes based on changes in pitch length were confirmed. When reproduced with 1 mW of left-handed circularly polarized light, there was a remarkable increase in reflectance in the information recording section, and reproduction was possible with an S/N of ~50 dB.
Note that when the incident power density was continuously changed from 25 to 65 mJ/cm ² , a continuous increase in reflectance was observed corresponding to the incident energy density. This also revealed the possibility of multilevel recording by multilevel control of incident energy. The recorded portion could be erased by heating at 70° C. or higher.

Example 4 Compound as polymer liquid crystal containing light absorber []
When the same light irradiation as in Example 3 was carried out on a medium having the same structure as in Practical Example 3 except that 1 was used, almost the same results as in Example 3 were obtained.

(Effects of the Invention) As described above, the present invention makes it possible to provide an erasable recording medium that is easy to manufacture, low cost, and can achieve a high S/N ratio of 60 dB.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of a recording medium according to the present invention. In the figure, 1...support substrate, 2...polymer liquid crystal including a light absorption layer, 3...protective film, 4...light beam.

Claims (1)

[Scope of Claims] 1. An optical recording layer made of a cholesteric polymer liquid crystal containing a light absorber, and the helical pitch p and refractive index n of the cholesteric polymer liquid crystal are approximately λ=λ=with respect to the reading light source wavelength λ. An optical recording medium characterized by being np. 2. The optical recording medium according to claim 1, wherein the light absorbent is added to a polymer material. 3. The optical recording medium according to claim 1, wherein the cholesteric polymer liquid crystal that touches the light absorber is a copolymer of a light absorbing molecule and a cholesteric liquid crystal molecule.